Though the temperature and time of heating and cooling processes are critical factors in the chemical and food processing industries, these two factors only provide partial information about heat transfer.
Taking the food processing industry as an example, a baker may prepare a batch of bread dough, divide it into two portions, and bake the two portions in different ovens at the same temperature and for the same time. Even if the design of each of the ovens is similar, the resulting bread products may be very different. One loaf might be light and moist on the inside and golden brown on the outside, yielding a very pleasant aroma. The other might be heavy and dry with a burnt crust, yielding a biting aroma. Most bakers have experienced the frustration of learning by trial and error the particular characteristics of each oven. Similar difficulties are encountered with cake, pizza, pie, quiche, and every other type of baked food product.
The problems of defining the characteristics of ovens and freezers is not limited to the food industry. Thermosetting of plastics or drying of paints, for example, can be accomplished in an oven or other heating apparatus. Freezing processes, particularly flash freezing processes, are impossible to fully characterize with the factors of time and temperature alone.
Most commercial baking ovens heat a food product by a combination of radiant and convection heating. Radiant heat transfer varies in proportion to the distance from the source to the product cubed or raised to the third power. Convection heat transfer rate varies with air velocity and air flow conditions, such as impingement. Temperature is only one factor.
Ovens with computer programs which vary the form or intensity of energy applied during a baking cycle need more analysis than just a temperature-time profile. A practical and very major problem in the development of baking profiles of food ovens is that the foods change during baking in unpredictable ways, making their use as "standards" difficult, if not impossible. A long felt need exists for a method and apparatus to standardize heat transfer processes. A long felt need also exists for an accurate method to record both the temperatures and the relative effectiveness of the process temperatures in heating or cooling.